Carbonaceous articles are widely used today in both the chemical and metallurgical industries because they exhibit exceptional properties at high temperatures. For example, graphite crucibles or molds exhibit good mechanical strength and excellent thermal stability when subjected to high casting temperatures.
However, a major deficiency of carbonaceous articles is that they rapidly oxidize and erode at high temperatures, i.e., above about 500.degree. C. This deficiency can be particularly troublesome in those instances where the articles must be periodically replaced as they are oxidized and consumed. The final cost of the product is therefore significantly increased.
It has already been proposed to employ oxidation prohibitive coatings on the surfaces of carbonaceous articles, such as carbon or graphite electrodes. These protective coatings have been composed of carbides, silicides, oxides and metals, for example. They have been applied to the articles using various known methods such as by electric arc, flame spraying, vacuum heating and the like.
U.S. Pat. No. 3,140,193 to Kane, for example, discloses a graphite electrode having a protective coating consisting of an inner layer made of porous silicon carbide and an outer layer composed of silicon metal. The inner layer is applied to the graphite by vacuum heating in a furnace at elevated temperatures. A problem with these coatings has been that they bond to the carbon or graphite substrate with difficulty and they are also expensive.
Thus, there is a need in this industry for improved oxidation prohibitive coatings for use on carbonaceous articles which avoid the difficulties of the prior art and provide very effective protection against oxidation even under severe or abusive conditions, and which are easy to apply, inexpensive and readily adaptable to full-scale production.